This invention relates to the field of integrated circuit fabrication. More particularly, this invention relates to projection alignment and leveling of integrated circuits in substrate form during photolithographic processing.
Integrated circuits are basically formed of a plurality of layers, where different features are formed in each of the various layers. The features in each of the various layers are typically formed using a photolithographic process. As a part of the process, a photolithographic mask is prepared with the desired image formed in the mask. The substrate on which the integrated circuit is to be formed is coated with a photosensitive material called photoresist, and the photoresist is exposed with a light that is passed through the mask. Thus, the image present in the mask is projected onto the photoresist coated substrate, thereby exposing portions of the photoresist to the light, and masking other portions of the photoresist from the light.
Depending on the type of photoresist used, either negative or positive, those portions of the photoresist that are exposed to the light either remain after the photoresist is developed, or are washed away during the developing process. After a hard bake to drive out solvents from the remaining photoresist, the patterned layer on the substrate is processed in some manner, such as being etched, and the photoresist is removed. A new layer is then deposited or otherwise formed, and the process repeats itself until wafer form processing of the integrated circuit is substantially completed.
There has been a tremendous effort throughout the history of integrated circuit technology to continually find ways to reduce the size of the devices and structures within the monolithic integrated circuits so fabricated. Many problems inherent with the shrinking geometries of integrated circuits have been identified and overcome over the years of such development.
For example, one problem has to do with the imaging process described above. As device size has become increasingly smaller, the precision with which the pattern is focused on the substrate becomes increasingly important. For example, a very large image can tolerate a relatively softer focus better than a very small image can, because the softer focus has a smaller overall effect on the size of the very large image. Thus, factors that didn""t create an issue with focus in the past, tend to now create problematic issues with focus.
As a specific example, as the layers of the integrated circuit are built up through front end processing, the surface of the substrate tends to become somewhat uneven, and more especially so at the edges of the substrate. This degree of surface topography tends to be sufficient to require adjustments in the leveling of the substrate during exposure, as the reticle is stepped across the surface of the substrate. By adjusting the level of the substrate with each reticle step, the surface of the substrate within that field of view can be properly imaged.
Leveling is typically accomplished with laser beam measurement systems that are disposed in positions such as at the corners of the exposure field, and which detect the height of the substrate at each such position. However, for those fields of view that are at the edge of the substrate, one or more of the measurement positions may be off the edge of the substrate, rendering a measurement at that position impossible. Additionally, because the topography of the substrate tends to be extremely variable near the outer edge of the substrate, a measurement taken very near the edge of the substrate may not be representative of the height of the substrate adjacent the measurement position.
What is needed, therefore, is system by which devices within image fields at the edge of the substrate can be properly leveled.
The above and other needs are met by a method for leveling an exposure field of view at a peripheral edge of a substrate. The field of view is aligned to a first position at the peripheral edge of the substrate, where the field of view has an inner edge and an outer edge, relative to the peripheral edge of the substrate. Whole device patterns within the field of view are identified, and the alignment of the field of view is altered to a second position so as to place the outer edge of the field of view adjacent the whole device patterns within the first field of view. Level measurement information from the field of view at the second position is acquired and stored. The field of view is realigned to the first position, and the substrate is leveled within the field of view at the first position using the level measurement information acquired from the field of view at the second position.
In this manner, whole device patterns disposed in a field of view at a first position near the peripheral edge of the substrate can be accurately leveled, by using leveling information that is acquired from a virtual second position that might overlap an adjacent exposure position on the substrate. The leveling information is acquired at the virtual second position, which does not hang off the edge of the substrate, and then this accurate leveling information is used to expose the field of view at the first position, thus enabling the accurate leveling of the whole device patterns within the field of view at the first position.
In various preferred embodiments, the level measurement information acquired from the field of view at the second position is obtained from at least one sensor disposed in the outer edge of the field of view, or more preferably from at least one sensor disposed in each of the outer edge of the field of view and the inner edge of the field of view. The step of acquiring level measurement information from the field of view at the second position preferably comprises setting an exposure energy at the second position to zero and initiating an exposure sequence. Preferably, the step of leveling the substrate within the field of view at the first position using the level measurement information acquired from the field of view at the second position comprises disabling active leveling at the first position and initiating an exposure sequence. The step of altering the alignment of the field of view to a second position preferably comprises shifting the field of view either vertically or horizontally.
Also described herein is a programmable substrate exposure tool having a memory containing program steps operable to instruct the programmable substrate exposure tool to perform the method as described above. Additionally described is a digital storage medium containing program steps operable to instruct a programmable substrate exposure tool to perform the method as described above.
According to another aspect of the invention there is described a method for exposing a substrate having a peripheral edge. Second positions on the substrate are exposed at a zero energy by aligning a field of view to a first position at the peripheral edge of the substrate, where the field of view has an inner edge and an outer edge, relative to the peripheral edge of the substrate. Whole device patterns within the field of view are identified. The alignment of the field of view is altered to a nearest of the second positions, so as to place the outer edge of the field of view adjacent the whole device patterns within the field of view. Level measurement information is acquired from the field of view at the nearest second position, and stored. The field of view is exposed (at zero energy) at the nearest second position.
Each first position on the substrate is exposed by realigning the field of view to the first position, and leveling the substrate within the field of view at the first position using the level measurement information acquired from the field of view at the nearest second position, and exposing the field of view at the first position. Third positions located interior to the first positions and second positions on the substrate are then exposed.
According to yet another aspect of the invention there is described a method for exposing an exposure field of view at a peripheral edge of a substrate. The field of view is aligned to a first position at the peripheral edge of the substrate, where the field of view has an inner edge and an outer edge, relative to the peripheral edge of the substrate. Whole device patterns are identified within the field of view, and the alignment of the field of view is altered to a second position so as to place the outer edge of the field of view adjacent the whole device patterns within the field of view. Level measurement information is acquired from the field of view at the second position, and stored. The field of view is realigned to the first position, and the substrate is leveled within the field of view at the first position using the level measurement information acquired from the field of view at the second position. The field of view is exposed at the first position.